Hydrogels are polymeric materials which are used in the manufacture of medical devices such as soft contact lenses, intraocular lenses, etc. These polymers expand considerably from their xerogel state during hydration. The change in total volume of a polymer article made from the copolymer during water absorption (hydration) depends on the nature and hydrophilicity of the monomers/polymers of the copolymer.
In the manufacture of articles requiring exact parameters, such as contact lenses and intraocular lenses, the swelling of the article during hydration can become a serious problem. This is because accurate production requires reasonably precise predictability of the dimensional swelling in order to set the manufacturing parameters. Also the swelling produces distortions and stresses in the hydrated polymer matrix, and hence the article.
In fact, substantial residual stresses and the resulting distortions can make the material unacceptable for its intended use.
In the production of contact lenses, etc., it is unusual to shape the product to its final form from the hydrated hydrogel due to production difficulties. In fact, it is simpler to machine or cast the product the xerogel state. Now, as the article is to be used in a hydrated state, the swelling during hydration of the polymer must also be accounted for in considering the dimensions of an article which is to be produced from the polymer in the non-hydrated or xerogel state.
Further if there are any variations in the degree of swelling from batch to batch or within a batch of material, the product will not be manufactured to the desired final dimensions. This may happen in the absence of changes in total water uptake if the ratio of the extent of swelling in the x, y or z directions changes in a fashion which compensates for the total volume increase.
Also at times there is a definite need to prepare materials from hydrogels that do not show any change in the volume during hydration. For example, U.S. Pat. No. 4,093,361 describes the preparation of a hydrogel with no net swelling during hydration. In this case the monomer was polymerized in the presence of a non-reactive water soluble neutral filler material. After polymerization was complete the neutral filler was washed out with a solvent, thereby leaving the final dimensions of the hydrogels articles unchanged. However, polymers made in this way suffer the disadvantage that parameters such as hardness become unsatisfactory and the mechanical properties (e.g. modulus, tear strength, max. elongation) of the resulting product are no longer optimum.
Furthermore, there can be a need to change the surface characteristics of hydrogels through surface treatments of the polymer in the xerogel state. Substantial volume changes in the hydrogel duping hydration can render these surface modifications useless due to the development of cracks and fissures in the surface.